The present invention relates generally to measurement instruments and more particularly to a system and apparatus for scanning and imaging a surface of a semiconductor or other type of workpiece.
In the semiconductor industry there is a continuing trend toward higher device densities. To achieve these high densities there have been, and continue to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such a high device packing density, smaller features sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as the corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photo lithographic processes as well as high resolution inspection instruments. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which, for example, a silicon wafer is coated uniformly with a radiation-sensitive film (e.g., a photoresist), and an exposing source (such as ultraviolet light, x-rays, or an electron beam) illuminates selected areas of the film surface through an intervening master template (e.g., a mask or reticle) to generate a particular pattern. The exposed pattern on the photoresist film is then developed with a solvent called a developer which makes the exposed pattern either soluble or insoluble depending on the type of photoresist (i.e., positive or negative resist). The soluble portions of the resist are then removed, thus leaving a photoresist mask corresponding to the desired pattern on the silicon wafer for further processing.
The trend toward higher device densities in the manufacture of semiconductor devices also requires higher resolution scanning and inspection instruments for analyzing various features of semiconductor devices. A measuring apparatus is required to inspect semiconductor devices in association with manufacturing production line quality control applications as well as with product research and development. The ability to scan and/or view particular features of a semiconductor workpiece allows for adjustment of manufacturing processes and design modifications in order to produce better. products, reduce defects, etc.
The features of interest in a semiconductor device may be topographic. Conventional instruments for measuring topographic features include Scanning Probe Microscopes. One form of a Scanning Probe Microscope is an Atomic Force Microscope (AFM), which is sometimes alternatively referred to as a Scanning Force Microscope (SFM). AFMs include a sensor with a spring-like cantilever rigidly mounted at one end and having a scanning tip at a free end. AFMs may operate in contacting and noncontacting modes. In the contacting mode, the tip of an AFM is placed in low force contact with a surface of a semiconductor wafer or other workpiece of interest. The workpiece is then displaced relative to the AFM in one or more directions in a plane (e.g., the tip contacts the workpiece in a Z axis while the workpiece is displaced in the X and/or Y directions), to effect a scanning of the workpiece surface. As surface contours or other topographic features are encountered by the tip during scanning, the cantilever deflects. The cantilever deflection is then measured, whereby the topography of the workpiece may be determined.
In non-contacting operation, the tip is held a short distance, typically 5 to 500 Angstroms, from the workpiece surface, and is deflected during scanning by various forces between the workpiece and the tip. Such forces may include magnetic, electrostatic, and van der Waals forces. In both the contacting and non-contacting modes, measurements of a workpiece topography or other characteristic features are obtained through measuring the deflection of the cantilever. Deflection of the cantilever may be measured using precisely aligned optical components coupled to deflection measurement circuitry, although other techniques are sometimes employed.
Another form of Scanning Probe Microscopes is a Scanning Tunneling Microscope (STM). Where a feature of interest is non-topographic, AFMs as well as STMs may be utilized used to measure the workpiece feature. Examples of non-topographic features include the detection of variations in conductivity of a semiconductor workpiece material. An AFM can be used to scan a workpiece in the non-contacting mode during which deflections in the cantilever are caused by variations in the workpiece conductivity or other property of interest. The deflections can be measured to provide a measurement of the feature. STMs include a conductive scanning tip which is held in close proximity (within approximately 5 Angstroms) to the workpiece. At this distance, the probability density function of electrons on the tip spatially overlap the probability density function of atoms on the workpiece. Consequently, a tunneling current flows between the workpiece surface and the tip if a suitable bias voltage is applied between the tip and the workpiece. The workpiece and tip are relatively displaced horizontally (in the X and/or Y directions) while the tip is held a constant vertical distance from the workpiece surface. The variations in the current can be measured to determine the changes in the workpiece surface.
In another mode of operation, an STM can be used to measure topography. The scanner moves the tip up and down while scanning in the X and/or Y directions and sensing the tunneling current. The STM attempts to maintain the distance between the tip and the surface constant (through moving the tip vertically in response to measured current fluctuations). The movements of the tip up and down can be correlated to the surface topography profile of a workpiece.
Other features of interest in a workpiece may be visual. For example, it may be desirable to scan only specific devices in a semiconductor wafer workpiece, such as transistors, conductors, and the like. While an AFM or STM scan of the entire wafer may yield the desired topographical or other feature information, this requires a great amount of time, where in some circumstances only a localized scan is needed. In addition, tip wear is increased in situations where entire wafers are scanned only to measure small features of interest. In these circumstances, a visual image of the wafer or other workpiece may be used to locate the feature or device of interest, and a local scan may then be performed using one or more of the above methods.
Some conventional measuring instruments include an optical microscope on top of the head assembly of an AFM. However, these microscopes do not have the high resolution necessary to identify and locate the tiny devices and other features of interest in today""s high device density semiconductor products. In addition, a visual image of the portion of a workpiece being scanned is unavailable to such microscopes because the cantilever and/or tip assembly of AFMs and STMs partially or completely block the view of the surface near the tip. Prior measuring devices have included optical microscopes laterally offset from the scanning location of an AFM. While the view of the optical microscope may be unobstructed, the optical microscope does not view the portion of the surface under the AMF tip. Other attempts include an AFM head for attachment directly to an optical microscope. However, the optical microscope lens head and the AFM cannot be used simultaneously to view the same portion of the workpiece surface.
A measuring system and apparatus is provided which overcomes or minimizes the problems and shortcomings of the prior art. The present invention provides a measuring apparatus used to obtain high resolution visual images of a scanned workpiece surface while scanning the surface using atomic force microscopy, scanning tunneling microscopy, or other related scanning technologies. This allows high resolution viewing of the surface of a workpiece directly below and proximate an AFM scanning tip during scanning operation of the AFM. A user may thus locate specific areas or features of interest on a workpiece surface visually while the AFM is scanning without the need to offset the AFM or to change heads in a conventional optical microscope. In addition, the invention provides for higher resolution visual imaging than previously available. The present invention thus provides a single instrument which may be used for both surface scanning measurements, as well as visual imaging, alone or in combination.
In accordance with one aspect of the present invention a system and apparatus are provided for measuring features on a workpiece which advantageously associate an optical sensor with a scanning probe microscope scanning assembly which can view the portion of the workpiece surface directly below and/or near the scanning tip. The system may further comprise a computer, display, camera, stereo microscope, or other optical processor for analyzing or viewing an image of the workpiece surface based on signals from the optical sensor. In this regard, the optical sensor may comprise a charge coupled device (CCD) or other solid state camera and may be associated with the cantilever and/or the tip. The optical sensor may further be incorporated directly into and/or fabricated on the cantilever or the tip.
In accordance with another aspect of the invention, a scanning tip assembly is provided for scanning a workpiece in a scanning probe microscope which may comprise an AFM or STM. The scanning tip assembly comprises a base and a cantilever assembly, with an optical sensor associated with the cantilever assembly. The optical sensor is adapted to provide a signal representative of the visual image of at least a portion of a workpiece and may further be fabricated on or integrated within the tip or cantilever. The invention thus allows the image to be obtained while an AFM or STM is scanning the workpiece portion. The optical sensor signal may be provided to a computer, camera, or other optical processor adapted to generate and/or display a visual image of the workpiece surface. In accordance with another aspect of the invention, the optical sensor may comprise a charge coupled device or other solid state camera and may be associated with the cantilever and/or the tip. Another aspect of the invention provides for fabrication of the optical sensor on the cantilever and/or the tip.
In accordance with yet another aspect of the present invention, a scanning tip assembly is provided for a scanning probe microscope having an optical fiber adapted to receive reflected light from a portion of a workpiece. This feature allows an optical sensor, camera, computer, stereo microscope, or other optical processor to receive the reflected light from the optical fiber for creating and/or displaying a visual image of the workpiece based on the reflected light. The scanning tip may be employed in an AFM or other scanning probe microscope, thereby providing simultaneous viewing and scanning of a workpiece surface. Another aspect of the invention provides for multiple optical fibers, enabling three dimensional viewing of topographical features of a workpiece surface using, for example, a stereo microscope or other optical processor. Further, the optical fiber may be provided with a lens in order to widen the field of view of the workpiece surface.
In accordance with still another aspect of the present invention, a measuring apparatus is provided comprising a scanning probe microscope having an optical fiber adapted to receive reflected light from a feature of a workpiece, and a camera or other optical processor associated with the optical fiber and adapted to generate a signal representing a visual image based on the reflected light from the feature of the workpiece. In addition, multiple optical fibers may be employed and providing light to a stereo microscope or other optical processor for generation of three dimensional visual images of a workpiece surface.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.